Here's the ultimate RPG grind experience. The game practically plays itself!

There's always Arcanum, for two reasons:

1) If you install the latest unofficial patch or the Level 127 hack (which the patch also contains) you can continue to level up indefinitely, instead of stopping at level 50. I recommend installing the patch, as it adds a small amount of content and fixes a huge number of bugs.

2) There's a hidden in-game quest, which when followed exactly, makes you a demigod among mortals. No links or spoilers here, but you can google "Arcanum Gods Quest" to find out more. I also recommend using a walkthrough, for reasons.

Demise: Ascension has level cap of 1499 iirc.

Not an RPG or remotely like Disgaea, but in no game have I felt the ramp up of power than in Bloodstained. It's totally a power fantasy if you put any effort into leveling up yourself or your skills. It's part of why it's so great.

Disappointed they removed the cap in a fan patch. Caps exist for a reason.

Anyway... in Morrowind you can amass so much money for training and so many levels that eventually nothing can ever touch you. Technically the same in Oblivion and Skyrim but leveling is much slower due to lack of infinite training.

And sometimes the reason is that the devs rushed the game to market before the company folded, without doing a full playtest (or releasing any official updates) to address balancing issues: specifically, that if you clean out dungeons and sewers, take on optional side quests, do a little exploring, and have a fair number of random encounters, it's possible to hit the cap before you're even halfway through the main quest. I can't speak for you, but that sucked all the fun out of the game for me.

imo Arcanum is a game one plays for the story and the setting. Many of the gameplay mechanics are highly flawed and the combat is barely serviceable, imo not the kind of game OP is looking for.

Final Fantasy II (PSP/mobile versions)
While it doesn't have proper levels, in these versions you can get so powerful even before finishing the first mission that you can kill the strongest boss in the game in its strongest form with just mashing the attack button. There should be videos on Youtube about FFII's powerlevelling.

Mary Skelter: Nightmares (original version, available here, on Steam and on the PS Vita).
While it can be hard by default, you can easily mass grind in two points of the game which makes the rest of the game considerably easier. Just be warned it might involve a bit of a bug. I also don't remember seeing videos for it, so here it goes the explanation:
For the mass grinding, you need to find the areas with propelers in the cemitery (5th floor, iirc) and the station grounds (don't remember which floor). When you find then, you'll notice you can't normally access the areas they affect due to the strong wind. Use something like the bow ability to unlock the map of this part with the strong wind and then set the auto movement option to go as far as you can into this area. The party will then get stuck in the same two squares, trying to move foward but being pushed back due to the wind right after. And while that happens, you can still get random encounters (exp+gold+items+blood crystals very easily). Just mash the confirm button when the battle starts and you should be able to win easily (at least after some grinding). Having cleared the dungeon helps, since you won't have to deal with those things.
It's also possible to do it in the Switch version (comes as a bonus with Mary Skelter 2), but both games are FAR HARDER and only the station grounds can still be used for that trick, since it's the only area in both games that still has propelers with 2 normal squares after the wind stops (one square means the party will hit the walls and will stop moving, so no final dungeon from Mary Skelter 2, even though it also has propelers).

Elminage Gothic: There's a trick where you can let demons call for help before you kill all but one, put the last one to sleep, have everybody you don't want to get the XP run away, then have a remaining character kill the last enemy. Very useful for bringing new characters up to speed.

Since somebody mentioned the non-RPG Bloodstained, I could mention that Timespinner has a mode which increases the level cap from 99 to 255, but you have to beat the game on Nightmare level cap 1 first (which is not as hard as it sounds, if you're playing New Game + rather than a fresh New Game).

Edit: Forgot one thing about Elminage Gothic: The final bonus dungeon is designed to take your characters from sub-100 level s to around level 400 or so. Thing is, enemies in that dungeon (specifically Archer Polete and Stargazer) can wipe out a party that high in level rather easily.

I think you're looking for Agarest.

There's a ton of RPGs this applies to, like Pillars of Eternity, Fallout 3, Outer Worlds and whatever else. It was very intentional in those games, to force you to have a specific character build. I don't know of any proof the idea in Arcanum wasn't the same.

I know some prefer no caps, but that's an argument to have during the development stage, not for fans to change years later.

I'm not sure I see the issue. Someone has to actively add that mod/fan patch to a game so it's not like they're going to accidentally over-level. And if you've played through the game several times, it may be a fun change to go all God Mode on it.

*edit* I guess if the player is a first-timer they may not realize that the cap removal could affect things, but they'd still have to grind to make it that high.

Nobody mentioned Baldur's Gate 2 yet? In that game you start powerful and the actual goal is becoming a god. In between is a lot of powergaming.

I think any of the BG and Icewind Dale game would lend themselves to powergaming, but not sure that's what he was looking for, since he mentions gaining levels through grinding. Still, these were the first ones I thought of when reading the OP.